Demo Abstract: Open-Access Full-Duplex Wirelessin the ORBIT Testbed

Tingjun Chen∗, Mahmood Baraani Dastjerdi∗, Guy Farkash∗, Jin Zhou†, Harish Krishnaswamy∗, Gil Zussman∗∗Electrical Engineering, Columbia University, New York, NY 10027, USA

†Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA{tingjun@ee., b.mahmood@, guy.farkash@, harish@ee., gil@ee.}columbia.edu, jinzhou@illinois.edu

Abstract—In order to support experimentation with full-duplex(FD) wireless, we integrated an open-access FD transceiver withthe ORBIT testbed [1]. In particular, an ORBIT node with aNational Instruments (NI)/Ettus Research Universal SoftwareRadio Peripheral (USRP) N210 software-defined radio (SDR) wasequipped with the Columbia FlexICoN Gen-1 customized RF self-interference (SI) canceller box. The RF canceller box includesan RF SI canceller that emulates an RFIC canceller, which isimplemented using discrete components. It achieves 40 dB RFSI cancellation across 5MHz bandwidth. This demonstrationpresents the design and implementation of the open-access FDtransceiver and the baseline program, as well as an example ofa remote FD experiment where 85 dB overall SI cancellation isachieved across both the RF and digital domains.

I. INTRODUCTION

Due to its potential to double network capacity at thephysical layer and to provide many other benefits at higherlayers, full-duplex (FD) wireless has drawn significant at-tention [2]. The major challenge associated with FD is theextremely strong self-interference (SI) on top of the desiredsignal, requiring powerful self-interference cancellation (SIC)across both the RF and digital domains.

Our work on FD transceivers/systems within the ColumbiaFlexICoN project [3] focuses on integrated circuit (IC) im-plementations that are appropriate for mobile and small-form-factor devices [4], [5]. We presented the FlexICoN Gen-1 FDtransceiver and an FD wireless link in [6], featuring 40 dB RFSIC across 5MHz. The implemented Gen-1 RF SI cancelleremulates its RFIC counterpart that we presented in [5].

There is no existing open-access wireless testbed with FD-capable nodes, which is crucial for experimental evaluations ofFD-related algorithms at the higher layers. Therefore, to allowthe broader community to experiment with FD wireless, weintegrated an improved version of the Gen-1 RF canceller pre-sented in [6] with a National Instruments (NI)/Ettus ResearchUSRP N210 SDR in the open-access ORBIT testbed [1]. Sinceinterfacing an RFIC canceller with an SDR presents numeroustechnical challenges, we implemented the RF canceller ona printed circuit board (PCB) to facilitate the cross-layeredexperiments with an SDR platform.

In this demonstration, we present our cross-layered (hard-ware and software) implementation of an open-access FDtransceiver integrated with the ORBIT testbed, including thedesign and implementation of the Gen-1 RF canceller box andan FD transceiver baseline program. The detailed description

Fig. 1: Block diagram of the implemented FD transceiver.

(a) (b)Fig. 2: (a) The Columbia FlexICoN Gen-1 RF canceller box, and (b) theopen-access FD transceiver installed in the ORBIT testbed.

of RF canceller box and instructions of the baseline programcan be found in [7]. We also demonstrate an example of aremote FD experiment, where 85 dB overall SIC is achievedacross the RF and digital domains. The detailed instructionsand code are available at [8], [9]. The implemented FDtransceiver and the baseline program, which can be furtherextended to more complicated communication and networkingscenarios, can facilitate further hands-on research in the areaof FD wireless, as discussed in [7].

II. INTEGRATION OF THE FLEXICON GEN-1 RFCANCELLER BOX WITH AN ORBIT NODE

Fig. 1 shows the block diagram of the implemented FDtransceiver, in which a Gen-1 RF canceller box (as depictedin Fig. 2(a)) is connected to an Apex II multi-band antenna (atthe ANT port) and a USRP (at the TX IN and RX OUT ports).Fig. 2(b) shows the FD transceiver installed in the ORBITtestbed. The RF SI suppression is achieved by the circulatorand the RF SI canceller in the Gen-1 RF canceller box, wherethe circulator has a TX/RX isolation of around 20 dB and theRF SI canceller can provide 20-30 dB RF SIC.

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Fig. 3: Measured TX/RX isolation of the RF canceller box with and withoutturning on the RF canceller. The RF canceller box with the circulator and theRF canceller provides 40 dB RF SIC across 5MHz bandwidth.

As Fig. 2(a) shows, the RF canceller box contains fourcomponents: (i) a frequency-flat amplitude- and phase-basedRF canceller, which is an improved version of that presentedin [6], (ii) a coaxial circulator, (iii) a custom-designed antennatuner, and (iv) a SUB-20 controller. Fig. 3 shows an exampleof the measured TX/RX isolation (measured between TX INand RX OUT ports of the RF canceller box), where 40 dB RFSIC is achieved across 5MHz bandwidth.

Specifically, the RF canceller is implemented using discretecomponents on a PCB and is optimized around 900MHzoperating frequency. The RF canceller taps a reference signalfrom the output of the power amplifier (PA) at the TX sideand adjusts its amplitude and phase. Then, SIC is performedat the input of the low-noise amplifier (LNA) at the RXside. For amplitude adjustment, a 7-bit SKY12343-364LFdigital attenuator is used, in which the attenuation can beadjusted within a 31.75 dB range with a resolution of 0.25 dB.For phase adjustment, a Mini-Circuits passive SPHSA-152+phase-shifter is used, which covers full 360 deg and is con-trolled by an 8-bit TI-DAC081S101 digital-to-analog converter(DAC). The attenuator and DAC are programmed through theSUB-20 controller serial-to-parallel interface (SPI).

We developed an FD transceiver baseline program contain-ing two parts which runs on the host PC (i.e., the yellowbox in Fig. 2(b)): (i) a GNU Radio program for the mainSDR application, and (ii) a C-based SUB-20 program forconfiguring the RF canceller box. The digital SIC algorithmis based on Volterra series and a least squares problem and issimilar to that presented in [10].

III. DEMONSTRATION

The demonstration setup contains the FD transceiver thattransmits and receives simultaneously at 900MHz carrierfrequency with 10MHz sampling rate. A laptop is used tologin into the ORBIT testbed through ssh to execute theexample FD experiment. Demo participants can observe thereal-time signal spectrum and the performance of RF SIC.

In the demonstration, the FD transceiver (node11-10)transmits a 2.5MHz signal stream with QPSK modulationscheme at an average TX power level of 0 dBm. Fig. 4(a)shows the power spectrum of the received signal at the FDtransceiver, where 85 dB overall SIC is achieved, where 43 dBis from the RF domain, 42 dB is from the digital SIC algo-rithm, and the SI signal is canceled to the receiver noise floor

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(b)Fig. 4: Power spectrum of the received signal at the FD transceiver with andwithout the desired signal. The TX power level is 0 dBm.

at −85 dBm.1 In addition, another ORBIT node (node13-8)with a USRP serves as a second radio that transmits a singletone with frequency offset 1MHz. Fig. 4(b) presents the powerspectrum of the signal received at the FD node after RF anddigital SIC. As Fig. 4(b) shows, the SI at the FD node, whichtransmits the 2.5MHz QPSK signal, is canceled to the USRPreceiver noise floor after SIC in the RF and digital domains,and the digital SIC algorithm introduces minimal SNR loss tothe desired signal (with frequency offset 1MHz).

ACKNOWLEDGMENTS

This work was supported in part by NSF grants ECCS-1547406 and CNS-1650685, DARPA RF-FPGA and SPARprograms, a Qualcomm Innovation Fellowship, Texas Instru-ments, and Intel. We thank Steven Alfano, Jelena Diakoniko-las, Aishwarya Rajen, Jinhui Song, Mingyan Yu for theircontributions to the project. We thank Ivan Seskar, JakubKolodziejski, and Prasanthi Maddala from WINLAB, RutgersUniversity, for their help on the integration with the ORBITtestbed. We also thank Kira Theuer and Kendall Ruiz from NIand the NI technical support team for their help.

REFERENCES

[1] “Open-access research testbed for next-generation wireless networks(ORBIT),” http://www.orbit-lab.org/.

[2] A. Sabharwal, P. Schniter, D. Guo, D. W. Bliss, S. Rangarajan, andR. Wichman, “In-band full-duplex wireless: Challenges and opportuni-ties,” IEEE J. Sel. Areas Commun., vol. 32, no. 9, pp. 1637–1652, 2014.

[3] “The Columbia FlexICoN project,” http://flexicon.ee.columbia.edu/.[4] J. Zhou, N. Reiskarimian, J. Diakonikolas, T. Dinc, T. Chen, G. Zuss-

man, and H. Krishnaswamy, “Integrated full duplex radios,” IEEECommun. Mag., vol. 55, no. 4, pp. 142–151, 2017.

[5] J. Zhou, A. Chakrabarti, P. Kinget, and H. Krishnaswamy, “Low-noise active cancellation of transmitter leakage and transmitter noise inbroadband wireless receivers for FDD/co-existence,” IEEE J. Solid-StateCircuits, vol. 49, no. 12, pp. 1–17, 2014.

[6] T. Chen, J. Zhou, N. Grimwood, R. Fogel, J. Maraševic, H. Krish-naswamy, and G. Zussman, “Demo: Full-duplex wireless based ona small-form-factor analog self-interference canceller,” in Proc. ACMMobiHoc’16, 2016.

[7] T. Chen, M. Baraani Dastjerdi, J. Zhou, H. Krishnaswamy, and G. Zuss-man, “Open-access full-duplex wireless in the ORBIT testbed,” arXivpreprint arXiv:1801.03069, 2018.

[8] “Open-access full-duplex wireless in the ORBIT testbed: Instructionsand code,” https://github.com/Wimnet/flexicon_orbit.

[9] “Tutorial: Full-duplex wireless in the ORBIT testbed,” http://www.orbit-lab.org/wiki/Tutorials/k0SDR/Tutorial25, 2017.

[10] D. Bharadia, E. McMilin, and S. Katti, “Full duplex radios,” in Proc.ACM SIGCOMM’13, 2013.

1This USRP receiver noise floor is limited by the existence of environmentalinterference at 900MHz frequency. The USRP has a true noise floor of around−95 dBm at the same receiver gain setting, when not connected to an antenna.